The present invention relates to a pressure sensor and a method for forming the pressure sensor, and more specifically to a pressure sensor empolying a semiconductor substrate and a method for forming the same.
Generally, it is well known to fabricate a pressure sensor by, for example, etching a semiconductor substrate.
Japanese Patent Application First Publication No. 2-132337 discloses a pressure sensor employing a semiconductor substrate which has a diaphragm portion.
An example of such kinds of pressure sensors conventionally proposed includes a silicon substrate having opposed surfaces each lying substantially in a (100) crystal plane. The substrate has a recess recessed inwardly from one of the opposed surfaces to form a diaphragm portion having a reduced thickness. The recess is formed of a generally rectangular shape in section by anisotropic etching to form side walls lying substantially in a (111) crystal plane. A piezoresistive element is disposed on the diaphragm portion, which is capable of sensing strain caused in response to a pressure, for instance fluid pressure, exerted on the diaphragm portion.
In the conventionally proposed process for fabricating the pressure sensor, it is important to position the piezoresistive element and the diaphragm portion in a precise manner with respect to each other in order to correctly detect a value of the pressure acting on the diaphragm portion. For the alignment, masks for the piezoresistive element and the diaphragm portion are placed on the opposite sides of the silicon substrate in alignment with each other by using a suitable equipment such as a double sided aligner. It is known that the mask alignment on the opposite sides of the substrate is less accurate than the mask alignment carried out on one of the opposite sides thereof. This is because the masks tends to be placed in misaligned positions due to an offset of optical axes in the double sided aligner, for example, an infrared radiation aligner, a reflection-type aligner and the like.
If the opposite surfaces of the silicon substrate which are cut in the cutting process of the silicon substrate are out of the (100) crystal plane due to errors in slicing, abrasion or the like, in the cutting process, then the recess will be formed in a position displaced from a given position. Therefore, the diaphragm portion cannot be disposed in an intended position in the silicon substrate. In such case, even though the masks are precisely aligned with each other, it is likely that the diaphragm portion and the piezoresistive element are displaced from the alignment position.
A magnitude of the strain which is sensed by the piezoresistive element, varies depending on the position relative to the diaphragm portion. Therefore, the displacement of the diaphragm portion and the piezoresistive element from the alignment position influences the magnitude of the strain sensed by the piezoresistive element, making it difficult to detect the pressure acting on the diaphragm portion with high accuracy.
Meanwhile, if the diaphragm portion and the piezoresistive element each having a large size are used, dispersion of the sensed magnitude of strain which is caused by the misalignment of the diaphragm portion and the piezoresistive element, may be reduced and thus the accuracy of the pressure detection can increase. However, in this case, the pressure sensor cannot be reduced in size as the whole.